Hydrous Oxides

Hydrous oxides are amorphous metal oxides, on the surface of which exist hydroxyl groups which are present as a necessity to terminate the structure (see Figure 1A). The general formula for a hydrous oxide is [M(n)O(n-x)/2(OH)x• wH2O]m, where the central cation, M, is n-valent (n is typically * 3). Most of the metals in the periodic table are able to form hydrous oxides which exhibit ion exchange properties. However, for the material to be applied as an ion exchanger, it must be stable under the conditions used for exchange. In particular, solubility can be a deciding factor in the utility of hydrous oxides; stability to pHs extending from strongly alkaline to strongly acidic may be necessary. Those hydrous oxides comprised of large, low valent cations or small, multivalent cations tend to be soluble, while those intermediate between the two extremes are stable. Typical examples of acid- and alkali-stable hydrous oxides are those of Alni, Ga111, In111, SiIV, SnIV, TiIV, ThIV, ZrIV, NbV, BiV, MoV and WVI. Many of the materials are amphoteric, that is, they can act as either cation or anion exchangers depending on, principally, the pH of the electrolyte solution and the basicity of the metal forming the hydrous oxide (the strength of the metal-oxygen bond relative to the oxygen-hydrogen bond).

The change of a commercial alumina from cation exchanger to anion exchanger with varying pH is shown in the chapter by Dyer (Figure 8). The am-photeric nature of hydrous oxides may be illustrated schematically thus:

Table 4 Changes in surface properties of phosphomolybdates and phosphotungstates upon ion exchange

Approximate composition Surface area by N2BET (m2 g 1)b Pore volume x 103 (cm3 g 1) Mean pore radius (nm)

ofHPA salta

HPMo, NaPMo,











Essentially nonporous

193 40 145

Essentially nonporous

128 90 163

Essentially nonporous

52 15 6

50 31 34

40 52 40

aPMo, PW and SiW represent (PMo12O40)3", (PW12O40)3" and (SiW12O40)4" respectively. The charge-balancing cation indicated is assumed to be fully exchanged into the HPA, although some variation of composition is inevitable. Note that the surface properties will vary slightly depending upon the preparation and exact composition of the HPA. bSurface area determined using the Brunauer, Emmett and Teller isotherm approach.

Cation exchange typically takes place in alkaline solution, while anion exchange is preferred in acidic solution. Dissociation of M-O-H near to its isoelec-tric point allows both exchange mechanisms to operate simultaneously.

Silica, the most common and extensively studied of the hydrous oxides, is a weakly acidic cation exchanger. The physical properties of silica, particularly the porosity and surface area, vary widely depending upon the method of preparation. Generally, multi-valent cations interact more strongly with the silica surface than do univalent ones, while in all cases the interactions are relatively weak and ion exchange is facile. Silica possesses between 0.5 and 0.8 hydroxyl groups per nm2 on its surface.

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